\documentclass{article}
\usepackage{amsfonts}

\input{preamble.tex}

\begin{document}

% 
%$\max$,,
%$$,
\begin{tikzpicture}[node distance=2.5cm]
  \tikzstyle{arrow}=[->,thick];
  \node (1) [] {$\btheta^\text{old}$};
  \node (2) [below left of=1] {$p(\bZ|\bX,\btheta^\text{old})$};
  \node (3) [below right of=2] {$\E_{\bZ}[\ln p(\bX,\bZ|\btheta)]=\displaystyle\sum_{\bZ}
    p(\bZ|\bX,\btheta^\text{old})\ln p(\bX,\bZ|\btheta)\Q (\btheta,\btheta^\text{old})$};
  \node (4) [below right of=1] {$\btheta^\text{new}=\text{arg} \max_{\btheta}\calq(\btheta,\btheta^\text{old})$};
  \draw [arrow] (1) -- (2);
  \draw [arrow] (2) -- (3);
  \draw [arrow] (3) -- (4);
  \draw [arrow] (4) -- (1);
\end{tikzpicture}
\end{document}